[DebugLine] Be more robust in geussing the path style
[DebugLine] Be more robust in geussing the path style

My previous change didn't fix the Windows bot. This patch is an attempt
to make guessing the path style more robust by first looking at the
compile dir and falling back to the actual file if that's unsuccessful.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@368772 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb